1. Field of the Invention
The present invention relates to an image processing apparatus for toning an input image signal.
2. Related Background Art
The assignee of the present invention has a Japanese application of a method for binarizing a digital image signal to form an image by a laser beam printer in which a digital image signal is converted to an analog signal and the converted analog signal is composed with a pattern signal such as a ramp (triangular) wave signal to generate a pulse width modulated binary signal in order to improve tonality.
FIG. 2 shows the above method. A digital video signal is latched in a latch 1 by a video clock VK for synchronization. The video clock VK is derived by frequency-dividing a master clock MK by a J-K flip-flop 5. The video signal is converted to an analog video signal AV by a D/A converter 2. An output of the D/A converter 2 is converted to a voltage level by a resistor 3, and it is supplied to one input terminal of a comparator 4. On the other hand, the master clock MK is frequency-divided by a period switching signal from a frequency divider 6, and it is further frequency-divided by a J-K flip-flop 7 to generate a clock signal SK having a duty factor of 50%. It has the same ratio of period as a ratio of frequency division in the frequency divider 6 with respect to the video clock VK. The clock SK is integrated by a ramp wave generator 8 to produce a ramp wave C which is supplied to the other input terminal of the comparator 4 for comparison with the analog video signal AV. The comparator 4 produces a pulse width modulated signal E based on the comparison result of the analog video signal and the ramp wave signal. The pulse width modulated signal E is supplied to a drive circuit 21 which on/off-modulates a semiconductor laser 17 in accordance with the pulse width modulated signal to emit a laser beam 11. The laser beam 11 emitted from the semiconductor laser 17 is scanned by a scanner 18 which comprises a rotary polygon mirror and a galvanometer. Numeral 19 denotes a lens for focusing the laser beam 11 to a spot on a photo-conductor 9, and numeral 20 denotes a mirror for deflecting a light path.
The photo-conductor 9 is an electro-photographic photo-conductor drum which is rotated in the direction of arrow A. The photo-conductor 9 is first uniformly charged by a charger 10 and the scanned by and exposed to the laser beam 11 which is on/off-modulated in accordance with the modulation signal, generally perpendicularly to the direction of rotation of the photo-conductor 9. An electrostatic latent image thus formed on the photo-conductor 9 is visualized by a developing unit 12.
A visible toner image formed on the photo-conductor 9 is transfered to a transfer material 14 by a transfer charger 13. The visible toner image transferred to the transfer material 14 is fixed by a fixing unit (not shown), and the toner remaining on the photo-conductor 9 after the transfer is removed by a cleaner 15. Then, the charges remaining on the photo-conductor 9 are discharged by a discharging light from a lamp 16. Then, the above process is repeated.
A relationship between a level of the analog video signal and a level of the ramp wave may be one of those shown in FIGS. 3(a), 3(b) and 3(c) depending on a type of desired image.
In FIG. 3(a), for example, if a relationship between a white level .alpha. of the analog video signal and a maximum value of the ramp wave varies for some reason such as thermal characteristic of the ramp wave generator, the tone in a low image density area will significantly vary even if the variation of the relationship is small. If a relationship between a black level .beta. of the analog video signal and a minimum value of the ramp wave varies, the tone in a high image density area will be significantly varied. Even if the relationship between the white and black levels of the analog video signal and the ramp wave does not essentially vary, the image will be significantly affected by a slight shift of the levels, and the correction thereof is difficult to attain even by a synchroscope. Further, even if the relationship between the white and black levels of the analog video signal and the ramp wave is kept constant, tones in the high image density area and the low image density area significantly vary by the variation and difference of laser beam power, laser beam response and E-V characteristic of the photo-conductor where the laser beam printer is used.
The toning of the input image signal is disclosed in the following U.S. patents and U.S. patent applications assigned to the assignee of the present invention.
U.S. Pat. No. 4,800,442 PA0 U.S. Ser. No. 225,644 filed on Jul. 27, 1988 PA0 U.S. Ser. No. 897,053 filed on Aug. 15, 1986 PA0 U.S. Ser. No. 898,096 filed on Aug. 20, 1986 PA0 U.S. Ser. No. 237,614 filed on Aug. 24, 1988 PA0 U.S. Ser. No. 224,255 filed on Jul. 26, 1988 PA0 U.S. Ser. No. 188,712 filed on Apr. 29, 1988 PA0 U.S. Pat. No. 4,763,199 PA0 U.S. Ser. No. 923,026 filed on Oct. 24, 1986 PA0 U.S. Ser. No. 932,030 filed on Nov. 18, 1986 PA0 U.S. Ser. No. 010,539 filed on Feb. 3, 1987 PA0 U.S. Ser. No. 235,761 filed on Aug. 23, 1988 PA0 U.S. Ser. No. 161,266 filed on Feb. 18, 1988 PA0 U.S. Ser. No. 282,896 filed on Dec. 12, 1988 PA0 U.S. Ser. No. 033,462 filed on Apr. 2, 1987 PA0 U.S. Ser. No. 090,238 filed on Aug. 25, 1987 PA0 U.S. Ser. No. 081,442 filed on Aug. 4, 1987 PA0 U.S. Ser. No. 162,303 filed on Feb. 29, 1988 PA0 U.S. Ser. No. 119,622 filed on Nov. 12, 1987